1. Field of the Invention
The present invention relates in general to an original illuminating apparatus and an image reader having the same. In particular, the invention is suitable for an apparatus, for use in digital copying machines, image scanners, multifunction printers or the like, for illuminating efficiently an original (image information) with high illuminance to read image information of the original at high speed.
2. Related Background Art
FIG. 5 is a schematic diagram showing the construction of a main portion of a conventional image reader.
In FIG. 5, reference numeral 2 designates an original stage glass member on which an original 1 is to be placed. Reference numeral 7 designates a carriage in which an original illuminating apparatus 3 for illuminating the original 1 which will be described later, reflecting mirrors 4a to 4e, an imaging lens 5 and a reading unit or reading means 6 are integrally accommodated. Then, the carriage 7 is scanned in a sub-scanning direction in the figure with a sub-scanning mechanism 8 such as a motor to read successively image information of the original 1.
The image information thus read is sent to a personal computer or the like as an external apparatus through an interface (not shown).
The luminous flux which has been obtained by reflecting light from the original 1 by the reflecting mirrors 4a to 4e is optically folded in the inside of the carriage 7. The imaging lens 5 is adapted to image the light from the original 1 on the reading unit (CCD) 6 which will be described later. The CCD (Charge Coupled Device) linear sensor 6 as the reading unit is constructed such that a plurality of light receiving elements are arranged in a main scanning direction as a direction perpendicular to the paper.
Along with the promotion of high speed operation of personal computers in recent years, the image scanners which can be operated at high speed have been greatly demanded from the market. In addition, in the digital copying machines as well, the demand for the high speed machines has been increased. Thus, under the circumstances, the necessity for promoting the high speed operation of the image readers has been increased.
In the above-mentioned construction, for the promotion of the high speed operation of the image readers, it is necessary to increase a quantity of light reaching the CCD 6. While in order to attain this, there is conceivable a method of improving reflectivity of the mirrors 4a to 4e and brightness of the imaging lens 5, almost mirrors have already reflectivity of about 95%, and hence a large improvement on the reflectivity is not expected. In addition, if brightness of the imaging lens 5 is doubled, then the lens structure becomes complicated, and the increase of about three or more times in cost is necessarily estimated.
Then, the necessity of increasing a quantity of light for illumination by the improvement on the original illuminating apparatus 3 has been increased. The description will hereinbelow be given with respect to the details of the original illuminating apparatus 3 for use in the conventional image reader shown in FIG. 5. FIG. 6 is a schematic view showing the construction of an example of a main portion of the conventional original stage glass member 2 and original illuminating apparatus.
A tubular light source 51 such as a cold cathode tube or an Xe gas tube illuminates an area 1a to be read of the original 1 placed on the original stage glass member 2 with assistance of a combination of reflecting plates 52 and 53 which are arranged across an optical axis 50 of the imaging lens 5 from each other, i.e., which are respectively arranged on the opposite side and rear face side of the tubular light source 51.
A light shielding plate 54 is arranged between the tubular light source 51 and the original stand glass member 2. Almost light which is emitted towards an area other than the area 1a to be read on the original 1 can be blocked off by the light shielding plate 54. Thus, the light shielding plate 54 is adapted to reduce a quantity of light, which is directly incident on the CCD 6, or incident on the area 1a to be read after having been diffusedly reflected by the original stage glass member 2 and other members.
Only one tubular light source 51 is provided in this prior art, and hence in order to enhance the illumination efficiency, the positions and shapes of the reflecting plates 52 and 53 are optimized. However, as described above, the higher speed operation is required, and hence it is necessary to increase a quantity of light all the more.
FIG. 7 is a schematic view showing the construction of a main portion of a second prior art of the original stage glass member 2 and the original illuminating apparatus.
In the prior art shown in FIG. 7, two tubular light sources each shown in FIG. 6, i.e., tubular light sources 51A and 51B and rear side reflecting plates 52 and 53 are arranged across or both sides of the reading optical axis 50 from each other so as to be symmetrical with respect to the reading optical axis 50. As a result, a quantity of light for illumination that is incident on the area 1a to be read of the original 1 is greatly increased, which makes the high speed reading operation possible.
In the original illuminating apparatus shown in FIG. 7, there occurs “the reflection” phenomenon in which as exemplified with broken lines in the figure, the light which has been emitted from the tubular light source 51A to be reflected in the vicinity of the areas 1a to be read is reflected by a surface 51Ba of the other tubular light source 51B to be incident on the areas 1a to be read again. “The reflection” phenomenon in the original illuminating apparatus shown in FIG. 7 occurs due to the construction in which the tubular light sources 51A and 51B are respectively arranged on the left and right sides so as to be symmetrical with respect to the reading optical axis 50. It is known that if “the reflection” phenomenon occurs, then the concentration of the read image becomes ununiform due to the concentration of the area other than the area 1a to be read so that the image quality is remarkably degraded.
While for blocking-off of the above-mentioned “reflection” phenomenon, the positions of the light shielding plates 54A and 54B which are respectively arranged above the tubular light sources 51A and 51B must be optimized, if the mounting accuracy and the like are taken into consideration, there is the possibility that even the necessary illuminating light is shielded off.